Rechargeable lithium batteries use materials from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. For an electrolyte, an organic solvent or polymer is used. Rechargeable lithium batteries produce electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon materials, which intercalate lithium ions reversibly, are extensively used instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems which are associated with the metallic lithium can be prevented while achieving relatively high energy density, as well as much improved cycle life. In particular, boron may be added to carbonaceous materials to produce boron-coated graphite (BOC) in order to increase the capacity of the carbonaceous materials.
For the positive material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiNiO2, LiNi1-xCoxO2 (0<x<1), and LiMnO2. Manganese-based materials such as LiMn2O4 and LiMnO2 are easier to prepare and less expensive than the other materials and are environmentally friendly. However, manganese-based materials have relatively low capacity. LiNiO2 is inexpensive and has a high capacity, but is difficult to prepare in the desired structure and is relatively less stable in the charged state causing a battery safety problem. LiCoO2 is relatively expensive, but widely used as it has good electrical conductivity and high cell voltage. Most commercially available rechargeable lithium batteries (at least about 95%) use LiCoO2 as the positive active material.
Although LiCoO2 exhibits good cycle life characteristics and good flat discharge profiles, there are still demands to improve electrochemical properties such as good cycle life and high power density.
One way to satisfy such a demand is to substitute a part of the Co from LiCoO2 with other materials. Sony studied LixCo1-yAlyO2 by doping about 1 to 5 percent by weight of Al2O3 into LiCoO2. A&TB (Asahi & Toshiba Battery Co.) studied a Sn-doped Co-based active material by substituting a part of Co from LiCoO2 with Sn.
Even though these studies have progressed, there are still demands for improving good thermal safety.